7 Answers
7

It's an identification number that will uniquely identify something. The idea being that that id number will be universally unique. Thus, no two things should have the same uuid. In fact, if you were to generate 10 trillion uuids, there would be something along the lines of a .00000006 chance of two uuids being the same.

Standardized identifiers

UUIDs are defined in RFC 4122. They're Universally Unique IDentifiers, that can be generated without the use of a centralized authority. There are four major types of UUIDs which are used in slightly different scenarios. All UUIDs are 128 bits in length, but are commonly represented as 32 hexadecimal characters separated by four hyphens.

Version 1 UUIDs, the most common, combine a MAC address and a timestamp to produce sufficient uniqueness. In the event of multiple UUIDs being generated fast enough that the timestamp doesn't increment before the next generation, the timestamp is manually incremented by 1. If no MAC address is available, or if its presence would be undesirable for privacy reasons, 6 random bytes sourced from a cryptographically secure random number generator may be used for the node ID instead.

Version 3 and Version 5 UUIDs, the least common, use the MD5 and SHA1 hash functions respectively, plus a namespace, plus an already unique data value to produce a unique ID. This can be used to generate a UUID from a URL for example.

Version 4 UUIDs, are simply 128 bits of random data, with some bit-twiddling to identify the UUID version and variant.

UUID collisions are extremely unlikely to happen, especially not in a single application space.

It's a very long string of bits that is supposed to be unique now and forever, i.e. no possible clash with any other UUID produced by you or anybody else in the world .

The way it works is simply using current timestamp, and an internet related unique property of the computer that generated it (like the IP address, which ought to be unique at the moment you're connected to the internet; or the MAC address, which is more low level, a hard-wired ID for your network card) is part of the bit string.

Originally every network card in the world has its own unique MAC address, but in later generations, you can change the MAC address through software, so it's not as much reliable as a unique ID anymore.

A UUID is a 128-bit number that is used to uniquely identify some entity. Depending on the specific mechanisms used, a UUID is guaranteed to be different or is, at least, extremely likely to be different from any other UUID generated. The UUID relies upon a combination of components to ensure uniqueness. A UUID contains a reference to the network address of the host that generated the UUID, a timestamp and a randomly generated component. Because the network address identifies a unique computer, and the timestamp is unique for each UUID generated from a particular host, those two components should sufficiently ensure uniqueness.

UUID stands for Universally Unique IDentifier.
It's a 128-bit value used for a unique identification in software development. UUID is the same as GUID (Microsoft) and is part of the Distributed Computing Environment (DCE), standardized by the Open Software Foundation (OSF).

As mentioned, they are intended to have a high likelihood of uniqueness over space and time and are computationally difficult to guess. It's generation is based on the current timestamp and the unique property of the workstation that generated the UUID.

I just want to add that it is better to use usUUID (Static windows identifiers).
For example if a computer user that relies, on a third party software like a screen reader for blind or low vision users, the other software (in this case the screen reder) will play better with unique identifiers!
After all how happy will you be if someone moves your car after you know the place you parked it at!!!